1. Field of the Invention
The present invention relates to a differential pressure flowmeter for measuring a flowrate of a fluid, and preferably, a differential pressure flowmeter is used for a flow controller for controlling a flowrate of a fluid and an apparatus for processing a substrate.
2. Description of the Background Art
Conventionally, in cleaning a semiconductor substrate (hereinafter, referred to as simply “substrate”), well known a technique where a dilute hydrochloric acid (HCI) is used instead of a pure water as a cleaning liquid, whereby preventing fine particles in the cleaning liquid from adhering to a surface of the substrate by a Coulomb force. Also, etching of the substrate is performed by using a dilute hydrofluoric acid (HF) or final cleaning of the substrate is performed by using a dilute acid solution (hydrochloric acid, hydrofluoric acid or the like).
A cleaning apparatus of the substrate uses a solution diluted a stock solution of the hydrochloric acid at less than or equal to 1/1000. For simplification or miniaturization or the like of a construction of an apparatus, this diluted solution is normally produced by a method (i.e., the so-called direct mixing method) in which a small amount of the undiluted solution of hydrochloric acid is directly injected into a tube for the pure water of the cleaning apparatus. In the cleaning apparatus, a flowrate of the hydrochloric acid injected into the pure water is measured by a flowmeter and by controlling the flowrate of the hydrochloric acid on the basis of an output from the flowmeter, the diluted solution is set at the desired concentration.
In the above case, employed is a differential pressure flowmeter where the flowrate is measured to measure a pressure difference in the front and back of an orifice plate disposed within a channel and Japanese Patent Application Laid Open Gazette No. 2000-283810 (Document 1) discloses a technique for improving measuring accuracy of this flowmeter. Also, U.S. Pat. No. 5,672,832 (Document 2) and U.S. Pat. No. 6,578,435 (Document 3) disclose a differential pressure flowmeter where a nozzle is utilized instead of an orifice plate. Japanese Patent Application Laid Open Gazette No. 2004-226142 (Document 4) and Japanese Patent Application Laid Open Gazette No. 2004-226144 (Document 5) disclose a differential pressure flowmeter where by measuring a pressure difference in both ends of a capillary, measurement of a very small flowrate is performed stably.
In producing the diluted solution, an extremely small amount of an undiluted solution needs to be injected into the pure water with high accuracy. For example, in a batch-type cleaning apparatus, a flowrate of the undiluted solution is normally less than or equal to 100 ml/min, this very small flowrate needs to be measured high accurately and controlled. In a single wafer-type cleaning apparatus, a flowrate of a stock (or undiluted) solution is set to be less than or equal to 10 ml/min.
Since the differential pressure flowmeters of Documents 1 to 3 make turbulent flow in the vicinity of the orifice plate or the nozzle and measure a flowrate, they are not suitable for measurement of a very small flowrate having a high possibility of a laminar flow. In the case of measuring a very small flowrate by these differential pressure flowmeters, an orifice or a nozzle with a very small diameter needs to be formed high accurately to obtain a significant pressure difference in the front and back of the orifice plate or the nozzle. This leads to not only increase in manufacturing costs of the differential pressure flowmeter but, when a flowrate is smaller, there is a possibility that the orifice or the like can not be formed in a desired size. Further, the orifice or the nozzle with the very small diameter may be blocked by foreign substances or there is a risk that cavitation may occur in the vicinity of the orifice or an outlet of the nozzle.
In the differential pressure flowmeters of Documents 4 and 5, a long capillary is used as a pressure loss part and assuming that a flow of a liquid in the capillary is laminar, a flowrate is obtained on the basis of an equation with respect to pressure loss in the laminar flow in a circular tube. This makes a diameter of the capillary relatively large and attempts to resolve problems of the differential pressure flowmeters of Documents 1 to 3. However, in the case where the flow is transitional or turbulent, the measuring accuracy of a flowrate decreases, and thus it is important to make a stable laminar flow. Also, since the flowrate is obtained by using the above equation concerning a straight circular tube in spite of using the capillary having a bending part actually, errors of measuring flowrate increase. Further, a stainless-steel capillary is joined to a capillary block by brazing, therefore formation or arrangement of the capillary is limited and also it is difficult to adjust a length of the capillary.